The present invention relates to magnetic tunnel junction sensors for reading signals recorded in a magnetic storage medium, and more particularly, this invention relates to improving the magnetoresistance of magnetic tunnel junction sensors.
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
FIG. 1 shows a typical spin valve sensor 100 (not drawn to scale) comprising end regions 104 and 106 separated by a central region 102. The central region 102 has defined edges and the end regions are contiguous with and abut the edges of the central region. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer 115. The magnetization of the pinned layer 120 is fixed through exchange coupling with an antiferromagnetic (AFM) 125. An underlayer 126 is positioned below the AFM layer 125.
The underlayer 126, or seed layer, is any layer deposited to modify the crystallographic texture or grain size of the subsequent layers, and may not be needed depending on the substrate. A variety of oxide and/or metal materials have been employed to construct underlayer 126 for improving the properties of spin valve sensors. Often, the underlayer 126 may be formed of tantalum (Ta), zirconium (Zr), hafnium (Hf), or yttrium (Y). Ideally, such layer comprises NiFeCr in order to further improve operational characteristics.
Free layer 110, spacer 115, pinned layer 120, the AFM layer 125, and the underlayer 126 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current Is from a current source 160 to the MR sensor 100. Sensor 170 is connected to leads 140 and 145 senses the change in the resistance due to changes induced in the free layer 110 by the external magnetic field (e.g., field generated by a data bit stored on a disk). IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the spin valve effect.
Another type of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetic moment fixed, or pinned, and the other ferromagnetic layer has its magnetic moment free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. IBM""s U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect.
FIG. 2 shows a prior art MTJ sensor 200 comprising a first electrode 204, a second electrode 202, and a tunnel barrier 215. The first electrode 204 comprises a pinned layer (pinned ferromagnetic layer) 220, an antiferromagnetic (AFM) layer 230, and a seed layer 240. The magnetization of the pinned layer 220 is fixed through exchange coupling with the AFM layer 230. The second electrode 202 comprises a free layer (free ferromagnetic layer) 210 and a cap layer 205. The free layer 210 is separated from the pinned layer 220 by a non-magnetic, electrically insulating tunnel barrier layer 215. In the absence of an external magnetic field, the free layer 210 has its magnetization oriented in the direction shown by arrow 212, that is, generally perpendicular to the magnetization direction of the pinned layer 220 shown by arrow 222 (tail of an arrow pointing into the plane of the paper). A first lead 260 and a second lead 265 formed in contact with first electrode 204 and second electrode 202, respectively, provide electrical connections for the flow of sensing current Is from a current source 270 to the MTJ sensor 200. A signal detector 280, typically including a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to the first and second leads 260 and 265 senses the change in resistance due to changes induced in the free layer 210 by the external magnetic field.
Studies have been conducted relating to the affect of the design of the free layer 210 on the magnetoresistance of MTJ sensors such as that of FIG. 2. In particular, the magnetoresistance of a MTJ sensor may be improved by reducing the thickness of the free layer 210 thereof. For more information on such studies, reference may be made to: A. Vedyayev, N. Ryzhanova, R. Vlutters, B. Dieny xe2x80x9cGiant tunnel magnetoresistance in multilayered meta/oxide structures comprising multiple quantum wellsxe2x80x9d J Phys: Condens Matter, 10 (1998), no, Page 5799-5805.
Unfortunately, MTJ sensor design requirements mandate that the free layer 210 be at least 30 A to afford proper operation. In particular, if the free layer 210 has a smaller thickness, the free layer 210 may become saturated, and the MTJ sensor may not exhibit linear operation.
There is thus a need for a MJT sensor system and method of manufacturing the same with a thin free layer to improve magnetoresistance, while preventing saturation and complying with design requirements.
A magnetic tunnel junction (MTJ) sensor system and a method for fabricating the same are provided. First provided is an antiferromagnetic (AFM) layer. A first ferromagnetic layer with a pinned magnetization is disposed adjacent to the AFM layer for serving as a pinned layer. Next included is a tunnel barrier layer adjacent to the first ferromagnetic layer, and a second ferromagnetic layer adjacent to the tunnel barrier layer. Adjacent to the second ferromagnetic layer is a spacer. A third ferromagnetic layer is positioned adjacent to the spacer for working in conjunction with the second ferromagnetic layer to serve as a free layer. The magnetization direction of the pinned layer is substantially perpendicular to the magnetization direction of the free layer at zero applied magnetic field. A cap layer resides adjacent to the third ferromagnetic layer. A thickness of the first ferromagnetic layer and second ferromagnetic layer is selected to achieve a resonant tunneling effect.
In one embodiment, the first ferromagnetic layer may include Fe. Further, the first ferromagnetic layer may include CoFe. The remaining ferromagnetic layers may include CoFe and/or NiFe. As an option, such remaining ferromagnetic layers may include a first CoFe layer, a second NiFe layer, and a third CoFe layer.
In another embodiment, the thickness of the first ferromagnetic layer and second ferromagnetic layer may each be less than 10 A. Further, the combined thickness of the second and third ferromagnetic layers may be greater than 30 A.
In still another embodiment, the tunnel barrier layer may be constructed from a non-magnetic metallic material. In particular, the tunnel barrier layer may include AlOx, AlN, and/or MgO. Further, the spacer may include Cu and/or CuOx. In use, the spacer decouples the second ferromagnetic layer and the third ferromagnetic layer.
Another magnetic tunnel junction (MTJ) sensor system is also provided. Included is an antiferromagnetic (AFM) layer. Adjacent to the AFM layer is a first ferromagnetic layer with a pinned magnetization for serving as a pinned layer. Associated therewith is a first tunnel barrier layer adjacent to the first ferromagnetic layer. A second ferromagnetic layer is positioned adjacent to the tunnel barrier layer. Instead of a spacer, a second tunnel barrier layer is disposed adjacent to the second ferromagnetic layer. A third ferromagnetic layer is positioned adjacent to the second tunnel barrier layer for working in conjunction with the second ferromagnetic layer to serve as a free layer. Similar to before, the magnetization direction of the pinned layer is substantially perpendicular to the magnetization direction of the free layer at zero applied magnetic field. Again, a cap layer is positioned adjacent to the third ferromagnetic layer, and a thickness of the first ferromagnetic layer and second ferromagnetic layer is selected to achieve a resonant tunneling effect.
Still another magnetic tunnel junction (MTJ) sensor system is also provided. Similar to before, an antiferromagnetic (AFM) layer, a first ferromagnetic layer, a first tunnel barrier layer, a second ferromagnetic layer, a second tunnel barrier layer, and a third ferromagnetic layer are provided. Still yet, a third tunnel barrier layer is positioned adjacent to the third ferromagnetic layer. Further, a fourth ferromagnetic layer is disposed adjacent to the third tunnel barrier layer for working in conjunction with the second ferromagnetic layer and the third ferromagnetic layer to serve as a free layer. The magnetization direction of the pinned layer is substantially perpendicular to the magnetization direction of the free layer at zero applied magnetic field. Again, a cap layer is disposed adjacent to the third ferromagnetic layer. Further, a thickness of the first ferromagnetic layer and second ferromagnetic layer is selected to achieve a resonant tunneling effect.